The enzyme phosphoribosylpyrophosphate (PRPP) synthetase is an important object of biochemical study for three reasons. (1) The enzyme catalyzes an unusual transfer of an intact pyrophosphoryl group. Comparison of the differences and similarities in its mechanism of catalysis with those of the much more numerous and universally important phosphotransferases and nucleotidyl transferases would illuminate our understanding of all three reaction types. (2) The product of this enzyme, PRPP, is required by all cells for biosynthesis of purine, pyrimidine, and pyridine nucleotides and of the amino acids histidine and tryptophan. It is important to understand how such a crucial, highly branched pathway is regulated. (3) The proper balance of PRPP metabolism is known to be essential to maintenance of normal nucleotide metabolism, especially purine metabolism. Imbalances have been shown to lead to certain forms of gouty hyperuricemia, neurological disorders, hemolytic amemias, and are possibly implicated in other disorders. This research projects physical, chemical, and kinetic studies of the structure, mechanism of catalysis, and regulation of the PRPP synthetase from Salmonella typhimurium. Binding of substrates and the allosteric inhibitor ADP will be examined by equilibrium dialysis and NMR line-broadening techniques. The structure of the enzyme subunits and native aggregation states will be examined. Mutants affected in PRPP synthetase have been and will be isolated and characterized to investigate the regulation of synthesis and physiological functioning of this enzyme in the transport and metabolism of nucleic acid precursors.